1. Field of the Invention
The present invention relates to a solid state image pickup device and, more specifically, to a solid state image pickup device capable of switching a mode for picking up still pictures and a mode for recording moving pictures.
2. Description of Related Art
A solid state image pickup device comprises a photoelectrical conversion element array and a pixel-data-reading-out control unit for controlling reading-out of pixel data from the photoelectrical conversion element array. The photoelectrical conversion element array is formed with a plurality of photoelectrical conversion elements (pixels) arranged in matrix. The photoelectrical conversion element array generates pixel data by performing photoelectrical conversion on optical images formed on a plurality of photoelectrical elements by incident light entered through an optical system. As for the photoelectrical conversion element array, the number of pixels has been dramatically increased due to epoch-making developments in the semiconductor technology in recent years. In the solid state image pickup device using such a photoelectrical conversion element array, the pixels are utilized for enabling to pickup still pictures and to record moving pictures. For picking up still pictures, it is carried out by utilizing the pixel data for the whole pixels on the photoelectrical conversion element array. This is referred to as a whole-pixel-reading-out mode, in which the pixel data read out from the photoelectrical conversion element array are outputted in order by a data unit of each pixel. Thereby, it enables to pickup highly fine still pictures.
Meanwhile, as for recording moving pictures, it is difficult to carry out recording through the whole-pixel-reading-out mode similar to the one used at the time of picking up the still pictures, since there is a limit in the operation speed of a digital signal processing unit and due to power consumption. Thus, for recording moving pictures, in general, pixel data processing is performed, in which pixels are thinned out for increasing the number of frames per unit time. Recording of moving pictures by thinning out the pixels as described above is generally referred to as a vertically/horizontally-mixed-pixel-reading-out mode.
The vertically/horizontally-mixed-pixel-reading-out mode will be described in detail. For the pixel data read out from the photoelectrical conversion element array, a plurality of pixel data are mixed in the vertical and horizontal directions with respect to the arrangement of the photoelectrical conversion elements (pixels). And the mixed pixel data is outputted as one unit of pixel data. Thereby, the number of frames per unit time is increased, so that it enables to achieve smooth and fast recording of moving pictures by the solid state image pickup device to which the photoelectrical conversion element array with high pixels (that is, a large number of pixels) is mounted.
Thinning out of the pixels and switching of the mixed-pixel-reading-out mode and the whole-pixel-reading-out mode as described above can be excellently achieved, especially, by a MOS image sensor, since it is possible to read out pixel data in any lines at will using signal lines, without having the MOS image sensor unlike CCD image sensors, to transfer electric potentials by transferring potential well.
The MOS image sensor is advantageous in respect that it can be operated with low voltage, bears less amount of current leak, has still larger numerical aperture compared to the CCD in the same size, has high sensitivity, can read out data easily compared to the CCD, etc. Especially, it is extremely advantageous in respect that it can select and read out pixels at will, and in terms of mixing the pixels.
In a solid state image pickup device comprising an imaging monitor such as a liquid crystal display unit, auto-focusing of an optical system is performed. In general, in the initial state at the time of supplying electric power, the subject is displayed in the imaging monitor in the moving picture mode. This is called a monitor mode. The monitor mode can be used for moving pictures, and the vertically/horizontally-mixed-pixel-reading-out mode in which the pixels are thinned out is applied thereto. In the monitor mode, when a shutter button is pressed for picking up a still picture, the auto-focus of the optical system is activated in the half-pressed state of the button, the shutter button is further pressed down (fully-pressed state) for releasing the shutter when in focus, and the mode is switched to the whole-pixel-reading-out mode for picking up a still picture.
An example of the auto-focus of the related art will be described by referring to FIG. 11.
Step S21 is in a standby state for picking up pictures right after a supply of electric power and it is set in the monitor mode. In step S22, the vertically/horizontally-mixed-pixel-reading-out mode is set. In step S23, it is judged whether it is for recording moving pictures or picking up still pictures. When a person taking pictures selects the mode for recording moving pictures, it is proceeded to step S24. When the person taking pictures selects the mode for picking up still pictures, it is proceeded to step S26. When proceeded to the step S24, the auto-focus is activated for driving the lens of the optical system and, in step S25, moving picture is recorded by the vertically/horizontally-mixed-pixel-reading-out mode through pressing a recording button. In the step S24, auto-focus is carried out by the pixel data which are mixed two-dimensionally in the vertical and horizontal directions. When proceeded to the step S26, the lens of the optical system is driven by rough auto-focus. In step S27, the half-pressed state of the shutter button is judged. In step S28, as in the same manner as that of the step S24, the lens of the optical system is driven by activating the auto-focus by the pixel data which are mixed two-dimensionally in the vertical and horizontal directions. When it is judged in step S29 that it is in focus, the whole-pixel-reading-out mode is set in step S30 and pickup of still pictures is carried out by the whole-pixel-reading-out mode by allowing the shutter button to be fully pressed.
In the related art, the auto-focus in the step S28 is carried out while being still in the vertically/horizontally-mixed-pixel-reading-out mode. In the related art, for picking up still pictures, the vertically/horizontally-mixed-pixel-reading-out mode which is similar to the one used for recording moving pictures is also used at the time of auto-focus control by the monitor mode. That is, the auto-focus is controlled by the pixel data in which pixels are thinned out by mixing the pixels. In other words, the base data for auto-focus is in the state with blank.
However, in general, in the auto-focus, high-band components in signal gain frequency characteristic of the pixel data are taken out through a band-pass filter, and it is judged to be in focus at the peak value.
FIG. 12A shows the frequency characteristic in the horizontal direction, in which the solid line is the frequency characteristic in the case where the pixels are not mixed, and the broken line is the frequency characteristic in the case where the pixels are mixed.
FIG. 12B shows a characteristic of the high-band horizontal high band-pass filter used for the auto-focus. When the pixels are mixed, the high-band information in the horizontal direction is lost.
As a result, in the related art in which the vertically/horizontally-mixed-pixel-reading-out mode is applied at the time of auto-focus control for picking up still pictures, it is not possible to achieve highly accurate auto-focus, following the pixel pitch.
Therefore, the object of the present invention is to provide a solid state image pickup device capable of achieving both modes for recording highly fine moving pictures with smooth movement by high pixels and for picking up highly fine still pictures through highly accurate auto-focus.